The present invention relates to a system for processing pulse-width modulated signals representing detected position, and which system is capable of detection with a high resolution of positions by finely dividing the signals representing detected position.
With a system for detecting the stroke position of a piston rod of a hydraulic cylinder, comprising magnetic scales each positioned axially at regular intervals and embedded in the surface of the piston rod and a pair of magnetic sensors mounted on the cylinder with 90 degrees phase shift therebetween, for example, the detection of stroke position consists of procedure steps of fetching outputs of the magnetic sensors varying with the movement of the piston rod, converting the outputs into pulse signals, and counting the number of the pulses. In this case, the signals of the magnetic sensors are sinusoidal wave signals, and if an arrangement is made such that these sinusoidal wave signals may generate pulse signals when they cross a zero point, positional detection may be achieved by the two sinusoidal wave signals which are out of phase with one another, having a resolution based on a quarter division of one pitch of the magnetic scale.
Japan Patent application No. 62-99203 describes further sub-division of each of the pitches of the magnetic scales for improvement of positional detection in accuracy.
In this invention, use is made of a sinusoidal wave having a frequency (e.g. 100 times) higher than that of a sinusoidal wave signal of a magnetic sensor so as to pulse-width modulate magnetic sensor signals, and subsequent counting of the pulse-width modulated signals makes it possible to carry out the detection of a position finely divided at a rate at which the pitch of the magnetic scale has been divided at regular intervals in correspondence with the frequency of a high frequency signal.
In this case, if the magnetic sensor signals are not shaped like a sinusoidal wave, but a highly linear one such as a triangular wave signal, the pitch can be divided at regular intervals to ensure the detection with a high degree of accuracy of a finely divided position, but since the signals of the magnetic sensor will in fact closely approximate to a sinusoidal wave, no division at uniform intervals may be achieved. It has been found, however, that use of a sinusoidal wave identical to a magnetic sensor signal as a pulsed-width modulating signal may cause improvement of the linearity during the pulse-width modulation.
However, due to the fact that the magnetic sensor signal in the form of a sinusoidal wave signal represents a high linearity in the vicinity of the zero point they will pass, but the curvature is small, and varies abruptly near the peak value of the signal, even the use of a high frequency sinusoidal wave signal as a modulating signal will result in a failure of linearity near the peak value, thereby causing a division at irregular intervals in association with unavoidable error in the detection of fine positions.
Japan Patent Application No. 63-98564 proposed the realization of the detection of fine positions correspondingly based on pitches divided at uniform intervals by procedure steps of dividing one cycle of each of two magnetic sensors into four quarter cycle sections, selecting a signal for a higher linear portion in each sections, and combining these selected signals into position signals.
In this instance, though the division of high accuracy can be achieved in each quarter cycle, when a section with a high linearity signal is switched to its next section, the signal tends to become discrete at a switching point, or to change its inclination, thereby impairing the linearity. This is caused by the fact that the center portions of amplitude of the magnetic sensor signal and modulating signal may shift when these signals are composed during the pulse-width modulation mode, or that the output property of the magnetic sensor signal may vary for each scale pitch due to any production fault of a magnetic scale.
This has resulted in variation of the divisional interval at a signal switching point with the consequential obtainment of no uniform resolution.
Thus, the object of the present invention is to provide a system for processing signals representing detected positions wherein a stable high resolution with an improved accuracy may be obtained in each of the scale pitches.